1. Field of the Invention
The present invention relates to a liquid crystal display device displaying images by both reflection display and transmission display, and a producing method thereof.
2. Description of the Related Art
Currently, liquid crystal display devices are used widely in electronic apparatuses such as a monitor, a projector, a mobile phone and a Personal Digital Assistant (hereinafter referred to as a “PDA”). Such a liquid crystal display device includes a reflection type, a transmission type and a transflective type.
A reflection type liquid crystal display device is configured so as to obtain reflection display by guiding surrounding light into an inside of a liquid crystal panel and reflecting the light by use of a reflective layer. Furthermore, a transmission type liquid crystal display device is configured so as to obtain transmission display by emitting light from a light source provided on a rear face side of a liquid crystal panel (hereinafter, explanation is given for a backlight as an example) to an outside via the liquid crystal panel.
Furthermore, in a transflective type liquid crystal display device, reflection display is hardly recognized in environment, where there is little surrounding light, such as nighttime, and therefore, only transmission display is observed substantially. In environment, where surrounding light is fluorescent lighting, such as indoor (hereinafter referred to as an “indoor environment”), reflection display using surrounding light and transmission display using light irradiated from a backlight are observed. Furthermore, in environment, where surrounding light is sunlight, such as outdoor (hereinafter referred to as an “outdoor environment”), it is difficult to recognize transmission display, and therefore, reflection display using the surrounding light is mainly observed. Thereby, it is possible to recognize display irrespective of surrounding brightness. That is, transflective type liquid crystal display devices are installed in mobile apparatuses such as a mobile phone, a PDA or a digital camera, since they are capable of displaying in any environment, both indoor and outdoor.
In such a transflective type liquid crystal display device, a liquid crystal panel has two kinds of display areas, that is, a reflection area used for reflection display and a transmission area used for transmission display. In the transmission area, light irradiated from a backlight passes through a color filter only once and is emitted to an outside. On the other hand, in the reflection area, surrounding light after passing through the color filter is reflected by a reflective layer, and passes the color filter again to be emitted to an outside. As described above, the number of times in which light passes through a color filter is different between the transmission area and the reflection area.
As a configuration of a color filter in such a transflective type liquid crystal display device, there is one, as a first method, that color filters of the same color material and the same thickness are simply formed in the reflection area and the transmission area respectively, as described in Japanese Kokai Publication 2000-111902 and Japanese Kokai Publication 2001-183646 as previous arts. In this configuration, however, reflection display becomes dark when a color filter securing a color reproduction range suitable for transmission display are used, since light passes through the color filter in a reflection area twice as described above. To cope with this problem, there is proposed the following method.
A second method is, as described in Japanese Kokai Publication 2000-111902, one in which a color filter of the same color material and the same thickness as that formed in a transmission area is formed in a reflection area and an unpigmented area is provided in the reflection area, whereby reflection display becomes bright even though the color filter is formed so as to have a color reproduction range suitable for transmission display.
A third method is, as described in Japanese Kokai Publication 2001-183646, one in which a color filter suitable for reflection display is formed in a reflection area, and a color filter suitable for transmission display is formed in a transmission area to thereby form a color filter having a color reproduction range suitable for the transmission display and make the reflection display bright.
A forth method is, as described in Japanese Kokai Publication 2002-296582, one in which a color filter in a reflection area is formed so as to be made of the same color material as that in a transmission area and have a film thickness thinner than that in the transmission area, to thereby make reflection display bright even though a color filter having a color reproduction range appropriate for transmission display is formed.
A color reproduction range of a color filter will be explained. In general, a color filter includes a plurality of filters corresponding to a plurality of primary colors, for example, three kinds of filters of red (R), green (G) and blue (B). In a liquid crystal display device, the amount of light passing through each of the filters of primary colors is adjusted independently, whereby various colors can be displayed.
In other words, a color of light emitted from the color filter is expressed by a color mixture of a plurality of primary colors. A color reproduction range of a color filter is an inside of a polygon obtained when chromaticity coordinates (x, y) of primary colors of light emitted from the color filter are shown on a chromaticity diagram of the CIE 1931 standard calorimetric system.
A color reproduction range of a color filter is defined as an area of the polygon calculated by using scales of a chromaticity diagram of the CIE 1931 standard colorimetric system. Therefore, a color obtained when the color reproduction range of a color filter is small has low color saturation. And, when the color reproduction range of a color filter is large, a color with high color saturation can be expressed, and therefore, display colors can be diversified. Furthermore, the larger the color reproduction range of a color filter becomes, the smaller the amount of light passing through the color filter becomes.
Both of the reflection display and the transmission display are observed in indoor environment. However, the second to fourth methods described above adopt a configuration that reflection display is made bright, and therefore display light emitted from the reflection area (hereinafter referred to as “reflection display light”) has a smaller color reproduction range. That is, when an observer observes both of the reflection display and the transmission display, the observer feels that the display is brighter than that in the case of observing the transmission display only and that the color reproduction range of a liquid crystal display device is smaller than that in the case of observing the transmission display only. This is because, when both of the reflection display and the transmission display are observed, the observer recognizes both of the reflection display light and display light emitted from the transmission area (hereinafter referred to as “transmission display light”), and because a color mixture of the reflection display light and the transmission display light is recognized as the color reproduction range of the liquid crystal display device.
Therefore, in the case where surrounding light is not so bright, reflection display light is little recognized, and therefore transmission display light is mainly recognized, and a difference in a color reproduction range of a liquid crystal display device is hardly recognized as compared with the case of observing transmission display only. However, as the surrounding light becomes brighter, the reflection display light is gradually recognized, and the color reproduction range of a liquid crystal display device is recognized differently as comparing with the case of observing the transmission display only.
On the other hand, in outdoor environment, both of the reflection display and the transmission display are observed. However, since the surrounding light is brighter, the reflection display light is mainly recognized and the transmission display light is little recognized. Since the reflection display light has a smaller color reproduction range as described above, the color reproduction range of a liquid crystal display device is different from that in the case of observing the transmission display only.
As described above, the color reproduction range of a liquid crystal display device is largely different between in the reflection display and in the transmission display, or from one environment of surrounding light to another.
A color reproduction range of a liquid crystal display device will be explained. A color reproduction range of a liquid crystal display device is a color reproduction range visually recognized by an observer when the observer actually observes a liquid crystal display device. That is, similar to the color reproduction range of a color filter, a color reproduction range of a liquid crystal display device is an inside of a polygon obtained when chromaticity coordinates (x, y) of primary colors of reflection display light or transmission display light are shown on a chromaticity diagram of the CIE 1931 standard calorimetric system. The color reproduction range of a liquid crystal display device is defined as an area of the polygon calculated by using scales of a chromaticity diagram of the CIE 1931 standard colorimetric system. Therefore, a color obtained when the color reproduction range of a liquid crystal display device is small, has low color saturation, and when the color reproduction range of a liquid crystal display device is large, a color with high color saturation can be expressed, and display colors can be diversified.
Furthermore, the color filter manufactured by the first method is considered as inappropriate for a transflective type liquid crystal display device by the following reasons. The color reproduction range of a color filter in the reflection area and that in the transmission area are same, but the color reproduction range of a liquid crystal display device becomes larger in the reflection display than that in the transmission display, in other words, the reflection display becomes dark, since the number of times light passes through the color filter is different between in the reflection display and in the transmission display.
The color reproduction range of a liquid crystal display device naturally differs between in the reflection display and in the transmission display.
In this way, in the first to fourth methods, the color reproduction range of a liquid crystal display device differs between in the reflection display and in the transmission display. The difference in the color reproduction ranges is not a large problem for display such as graphic display and character display in which colors is not important. However, in recent years, since there arises a necessity for performing delicate color image display such as TV image of natural picture or photograph in a mobile apparatus, the difference gradually becomes a major problem.